The present invention generally relates to a method and an apparatus for end point detection in a chemical mechanical polishing process by using two laser beams and more particularly, relates to a method and an apparatus for end point detection in a chemical mechanical polishing process by using two laser beams that have different wavelengths or different incident angles through one or two windows provided in a polishing pad.
In the fabrication of semiconductor devices from a silicon wafer, a variety of semiconductor processing equipment and tools are utilized. One of those processing tools is used for polishing thin, flat semiconductor wafers to obtain a planarized surface. A planarized surface is highly desirable on a shadow trench isolation (STI) layer, on an inter-layer dielectric (ILD) or on an inter-metal dielectric (IMD) layer which are frequently used in memory devices. The planarization process is important since it enables the use of a high resolution lithographic process to fabricate the next level circuit. The accuracy of a high resolution lithographic process can be achieved only when the process is carried out on a substantially flat surface. The planarization process is therefore an important processing step in the fabrication of semiconductor devices.
A global planarization process can be carried out by a technique known as chemical mechanical polishing or CMP. The process has been widely used on ILD or IMD layers in fabricating modern semiconductor devices. A CMP process is performed by using a rotating platen in combination with a pneumatically actuated polishing head. The process is used primarily for polishing the front surface or the device surface of a semiconductor wafer for achieving planarization and for preparation of the next level processing. A wafer is frequently planarized one or more times during a fabrication process in order for the top surface of the wafer to be as flat as possible. A wafer can be polished in a CMP apparatus by being placed on a carrier and pressed face down on a polishing pad covered with a slurry of colloidal silica or aluminum.
A polishing pad used on a rotating platen is typically constructed in two layers overlying a platen with a resilient layer as an outer layer of the pad. The layers are typically made of a polymeric material such as polyurethane and may include a filler for controlling the dimensional stability of the layers. A polishing pad is typically made several times the diameter of a wafer while the wafer is kept off-center on the pad in order to prevent polishing a non-planar surface onto the wafer. The wafer itself is also rotated during the polishing process to prevent polishing a tapered profile onto the wafer surface. The axis or rotation of the wafer and the axis of rotation of the pad are deliberately not collinear, however, the two axes must be parallel. It is known that uniformity in wafer polishing by a CMP process is a function of pressure, velocity and concentration of the slurry used.
A CMP process is frequently used in the planarization of an ILD or IMD layer on a semiconductor device. Such layers are typically formed of a dielectric material. A most popular dielectric material for such usage is silicon oxide. In a process for polishing a dielectric layer, the goal is to remove typography and yet maintain good uniformity across the entire wafer. The amount of the dielectric material removed is normally between about 5000 xc3x85 and about 10,000 xc3x85. The uniformity requirement for ILD or IMD polishing is very stringent since non-uniform dielectric films lead to poor lithography and resulting window etching or plug formation difficulties. The CMP process has also been applied to polishing metals, for instance, in tungsten plug formation and in embedded structures. A metal polishing process involves a polishing chemistry that is significantly different than that required for oxide polishing.
The important component needed in a CMP process is an automated rotating polishing platen and a wafer holder, which both exert a pressure on the wafer and rotate the wafer independently of the rotation of the platen. The polishing or the removal of surface layers is accomplished by a polishing slurry consisting mainly of colloidal silica suspended in deionized water or KOH solution. The slurry is frequently fed by an automatic slurry feeding system in order to ensure the uniform wetting of the polishing pad and the proper delivery and recovery of the slurry. For a high volume wafer fabrication process, automated wafer loading/unloading and a cassette handler are also included in a CMP apparatus.
As the name implies, a CMP process executes a microscopic action of polishing by both chemical and mechanical means. While the exact mechanism for material removal of an oxide layer is not known, it is hypothesized that the surface layer of silicon oxide is removed by a series of chemical reactions which involve the formation of hydrogen bonds with the oxide surface of both the wafer and the slurry particles in a hydrogenation reaction; the formation of hydrogen bonds between the wafer and the slurry; the formation of molecular bonds between the wafer and the slurry; and finally, the breaking of the oxide bond with the wafer or the slurry surface when the slurry particle moves away from the wafer surface. It is generally recognized that the CMP polishing process is not a mechanical abrasion process of slurry against a wafer surface.
While the CMP process provides a number of advantages over the traditional mechanical abrasion type polishing process, a serious drawback for the CMP process is the difficulty in end point detection. The CMP process is frequently carried out without a clear signal about when the process is completed by using only empirical polishing rates and time polish. Since the calculation of polish time required based on empirical polishing rates is frequently inaccurate, the empirical method fails frequently resulting in serious yield drops. Attempts have been made to utilize an end point mechanism including those of capacitance measurements and optical measurements. However, none of these techniques have been proven to be satisfactory in achieving accurate control of the dielectric layer removed.
Another method for achieving end point detection is marketed by the Applied Material Corporation of Santa Clara, Calif. in a Mirra(copyright) CMP device. In the Mirra(copyright) device, a system of in-situ rate monitor (ISRM) is provided to determine end point by the concept of a periodic change of optical interference. In the Mirra(copyright) device, signals received from a patterned wafer surface are processed by digital filtering algorithms by a PC programmable filter such that an optical interference intensity changes periodically with the thicknesses of removed surface material. For instance, a built-in laser source which is fixed at 6,700 xc3x85 wavelength is utilized to cause interference at a wafer surface and thus producing a waveform received by a laser detector. The waveform generated by such a technique is shown in FIG. 1.
FIG. 1 illustrates four cycles of a waveform with each cycle corresponds to a removed material layer thickness of approximately 2437 xc3x85. The technique is adequate to detect an end point in a polishing process wherein only a relatively thin layer, for instance, of only 2000 xc3x85 is removed. When a large thickness of material such as an IMD oxide layer having a thickness of at least 4000 xc3x85 is to be removed, the method frequently produces faulty results since the laser detector cannot distinguish which one of the waveform cycles the end point falls on. The wafer surface can therefore be either overpolished or under-polished by 2400 xc3x85 thickness. In other words, it is difficult for an operator to properly set a xe2x80x9cwindowxe2x80x9d of the polishing process to accurately control the thickness of the layer to be removed.
FIG. 2 illustrates a cross-sectional view of a conventional CMP apparatus such as supplied by the Applied Materials Corporation of an in-situ rate monitor system 10. In the ISRM system, a platen is equipped with a laser source capable of generating laser emissions and receiving signals from a patterned wafer surface such that the signals are processed by digital filtering algorithms by a PC programmable filter and that an optical interference intensity changes periodically with the thicknesses of removed surface material. A plane view of the conventional CMP apparatus 10 that has a wafer sample 16 positioned on a rotating platen 12 and window 40 in the platen is shown in FIG. 3.
In the apparatus 10 shown in FIG. 2, a polishing platen 12 which is intimately joined to a polishing pad 14 is used as the rotating platen in the CMP apparatus 10. The rotating platen 12 is equipped with a laser emitter, or a laser generating device 20 which includes a semiconductor diode (not shown) capable of generating laser emissions at a predetermined wavelength. As shown in FIG. 2, laser emission 22 is generated by the semiconductor diode at a desirable frequency. A semiconductor wafer 16 which consists of an oxide coating layer 18 overlying a base material layer 28 is shown. The base material layer be formed of any suitable materials such as silicon, polysilicon or metal. The semiconductor wafer 16 is pressed onto a rotating platen 12 such that a top surface 32 of the oxide layer 18 intimately contacts and frictionally engages a top surface 34 of the polishing pad 14. Laser emission 22 from the laser emitter 20 irradiates onto surface 32 of the oxide layer 18 through a window 40 that is provided in the polishing pad 14. A plane view of the window 40 is shown in FIG. 3. The laser emission 22 from the laser emitter 20 are partially reflected by the oxide surface 32 into reflected beam 36. Part of the laser beam 22 penetrates into the oxide layer 18 and are then reflected by the interface 50 formed between the oxide layer 18 and the base layer 28. The reflected beams are then deflected at the oxide surface 32 into laser beam 56 to be received by the laser detector 30.
The traditional technique of ISRM is only adequate for the detection of end point in a polishing process wherein only a relatively thin layer of material is removed. When a larger thickness of material on a semiconductor structure, such as an IMD oxide layer that has a thickness of 4000 xc3x85 or larger is to be removed, the conventional ISRM technique frequently produces faulty results since the laser detection device cannot distinguish which one of the wave form cycles that the end point falls on. As a result, it is quite possible that the wafer surface can be either over-polished or under-polished by a thickness as large as 2400 xc3x85. As a result, it is difficult for an operator to properly set a xe2x80x9cwindowxe2x80x9d for the polishing process in order to accurately control the thickness of the layer to be removed.
It is therefore an object of the present invention to provide a method for determining end point in a CMP process utilizing an optical interference technique that does not have the drawbacks or the shortcomings of the conventional methods.
It is another object of the present invention to provide a method for determining an end point in a CMP process by utilizing two laser emitters each having a different wavelength for reflecting off the surface of a wafer.
It is a further object of the present invention to provide a method for determining an end point in a CMP process by utilizing two laser emitters each having a different incident angle for reflecting off the surface of a polished wafer.
It is another further object of the present invention to provide a method for determining an end point in a CMP process wherein laser emissions having different wavelengths are utilized to produce a constructive wavelength interference pattern.
It is still another object of the present invention to provide a method for determining an end point in a CMP process by utilizing two laser emitters each emitting a laser emission having a different wavelength than the other wherein the difference between the two wavelengths is at least 50 nm.
It is yet another object of the present invention to provide a method for determining an end point in a CMP process by utilizing two laser emitters each having different incident angles for reflecting off a wafer surface wherein the difference in the incident angle is at least 2xc2x0.
It is still another further object of the present invention to provide an apparatus for determining an end point in a CMP process that is equipped with two laser beams wherein each of the beams emits a laser emission at a different wavelength.
It is yet another further object of the present invention to provide an apparatus for determining an end point in a CMP process that is equipped with two laser beams each emitting a laser emission at a different incident angle to a wafer surface.
In accordance with the present invention, a method and an apparatus for conducting an end point detection in a chemical mechanical polishing process by utilizing two laser beams are disclosed.
In a preferred embodiment, a method for determining an end point in a CMP process by using two laser beams can be carried out by the operating steps of providing a wafer surface, polishing the wafer surface by a polishing pad, directing a first laser beam at the wafer surface producing a first reflected beam, directing a second laser beam at the wafer surface producing a second reflective beam, collecting and analyzing the first and second reflective beams by a detector for determining an end point of the CMP process.
In the method for determining an end point in a CMP process by using two laser beams, the first and second laser beams may have different incident angles. The first and second laser beams may have different wavelengths. The method may further include the step of forming a constructive interference when the first reflective beam and the second reflective beam are collected and analyzed by the detector. The first laser beam and the second laser beam are directed at the wafer surface through a single window formed in the polishing pad. The first laser beam and the second laser beam may be directed at the wafer surface through two separate windows formed in the polishing pad. The first laser beam may have a wavelength smaller than 650 nm and the second laser beam may have a wavelength larger than 700 nm. The first laser beam may have a wavelength that is different than a wavelength of the second laser beam by at least 50 nm.
In another preferred embodiment, a method for determining an end point in a chemical mechanical polishing process by using two laser beams that have different incident angles can be carried out by the steps of providing a wafer surface to be polished, providing a polishing platen that has a polishing pad installed thereon, installing at least two laser emitters and a laser detector in the polishing platen, engaging the wafer surface and a top surface of the polishing pad intimately together while the wafer and the polishing pad are rotated in opposite directions, detecting a first laser beam from one of the at least two laser emitters at the wafer surface at a first incident angle producing a first reflective beam, directing a second laser beam from the other one of the at least two laser emitters at the wafer surface at a second incident angle different than the first incident angle producing a second reflective beam, and receiving the first reflective beam and the second reflective beam into a laser detector and forming a constructive interference for predicting an end point of the CMP process.
The method for determining an end point in a CMP process by using two laser beams that have different incident angles may further include the step of directing a second laser beam at the wafer surface at a second incident angle that is different by at least 2xc2x0 from the first incident angle producing a second reflective beam. The method may further include the step of directing the first laser beam and the second laser beam from the at least two laser emitters through a single window provided in the polishing pad. The method may further include the step of providing the first and second laser beams with substantially the same wavelength. The method may further include the step of providing the first and the second laser beams with a wavelength in the range between about 100 nm and about 10,000 nm.
The present invention is further directed to an apparatus for determining an end point in a chemical mechanical polishing process that is equipped with two laser beams including a wafer that has a top surface to be polished, a polishing platen that has a polishing pad installed thereon, at least two laser emitters and a laser detector installed therein, means for intimately engaging the top surface of the wafer to a polishing surface of the polishing pad, means for rotating the wafer and the polishing platen in opposite directions, at least one window in the polishing pad for laser beam emitted by the at least two laser emitters to go therethrough, a first laser beam from one of the at least two laser emitters projected at the top surface of the wafer producing a first reflective beam, and a second laser beam from the other one of the at least two laser emitters projected at the top surface of the wafer producing a second reflected beam, the first reflective beam and the second reflective beam are received by the laser detector to produce a constructive interference spectrum indicative of an end point of the CMP process.
In the apparatus for determining an end point in a CMP process equipped with two laser beams, the first laser beam has a different wavelength than the second laser beam. The first laser beam may have a wavelength smaller than 650 nm and the second laser beam may have a wavelength larger than 700 nm. The first laser beam may have a different incident angle on the top surface of the wafer than the second laser beam. The first laser beam may have an incident angle that is at least 2xc2x0 different than an incident angle of the second laser beam. The at least one window may include two windows in the polishing pad for the at least two laser beams to penetrate therethrough.